Ultrasonic surgical instruments have been available since the early 1970's. One early example is the Cavitron Phaco-Emulsifier, a Cavitron trademark, which provided an apparatus for eye surgery without tip vibration amplitude control. The vibration tip amplitudes were low and the load reflected back to the handpiece during surgery was a small percentage of the total power provided by the power supply used in the apparatus.
With the arrival of the Cavitron Ultrasonic Surgical Aspirator in 1976, the surgical tip amplitude was increased some five fold and the electrical power to drive the tip was approximately doubled. In addition, the tip amplitude was controlled so that it could remain constant for the full power range available from the power supply in the apparatus. This yielded an effective ultrasonic surgical instrument which was powerful enough to fragment and remove a broad range of tissues, from very soft parenchymal tissue to more fibrous and elastic membranes.
The surgeon was then faced with a dilemma. For most effective operation of the ultrasonic instrument, the amplitude of vibration needed to be near the maximum available. However, at high amplitude settings, the selectivity of tissue removal, i.e. the ability of the instrument to discriminate between high and low water content tissues, was diminished. As a result, the ultrasonic surgical instrument easily fragmented tissue that the surgeon wanted to remove but also fragmented tissue that was not intended to be removed.
One solution to this problem is proposed by U.S. Pat. No. 4,827,911. In this patent an ultrasonic surgical instrument is described wherein the ultrasonic tip is activated in a pulsed manner. This reduces the average energy into the tissue while providing a high amplitude tip motion for effective tissue fragmentation.
However, in recent years slender surgical tips started to be made available. Such tips are advantageous for many minimal invasive procedures. To counteract the significant loads presented to the power supplies used in the ultrasonic instruments and employing the slender tips, manufacturers have increased the maximum available power so that the ultrasonic tips will not stall or worse, have a reduced amplitude, which slows down the surgical procedure, increases the frictional heat transmitted to the patient, and heats the tip so that its functional life is reduced.
Hence, increasing the maximum available power does not come without problems. The increased available power, independent of the desired tip amplitude, decreases the sensitivity of the surgical instrument and can result in unwanted damage to surrounding tissue, perforation of tissue in front of the tip and reduced protection to the patient.